The present invention relates generally to protective relaying in a power transmission and/or distribution system, and more particularly to a method for use in connection with a protective relay or like device to accurately determine fault direction in a capacitance-compensated line of such a power system.
In a power distribution system, electrical transmission lines and power generation equipment must be protected against faults and consequent short circuits. Otherwise, such faults and short circuits can cause a collapse of the system, equipment damage, and/or personal injury. Accordingly, and as shown in FIG. 1, a typical power system employs a protective relay at each end of a transmission line to monitor system conditions on and adjacent such protected transmission line, to sense faults and short circuits on and adjacent such protected line, and to appropriately isolate such faults and short circuits from the remainder of the power system by tripping pre-positioned circuit breakers on such protected line.
As seen, a typical power system can be connected over hundreds of miles and include multiple power generators (generator S, generator R) at different locations. Transmission lines (the main horizontal lines in FIG. 1) distribute power from the generators to secondary lines or buses (the main vertical lines in FIG. 1), and such buses eventually lead to power loads such as motors, houses, office buildings, manufacturing facilities, industrial plants, cities, etc. Importantly, relays and circuit breakers are appropriately positioned at each end of a protected transmission line to perform the isolating function described above.
A modern protective relay typically measures voltage and current waveforms measured at the respective end of a corresponding protected line, and employs a microprocessor and/or digital signal processor (DSP) to process the measured waveforms. As used herein, the term xe2x80x98transmission linexe2x80x99 includes any type of electrical conductor, such as a high power conductor, a feeder, etc. Based on the processed waveforms, the protective relay can then decide whether to trip an associated breaker, thereby isolating a portion of the power system.
In particular, and referring now to FIG. 1A, it is seen that a typical protective relay 10 samples voltage and current waveforms VA, VB, VC, IA, IB, IC from each phase (A-C) of a three phase line 12 at a particular point along such line 12. Of course, the line 12 may have greater or lesser numbers of phases, and greater or lesser numbers of phases in a line 12 may be sampled. The sampled waveforms are stored in a memory 14 and may then be retrieved and appropriately operated on by a processor or DSP 16 to produce estimated voltage, current, and impedance phasors. As should be understood, such phasors merely express such values in complex form according to magnitude and angle. As should also be understood, such impedance phasors in particular are employed to determine whether a fault condition exists, and if so to determine fault direction and estimate fault location, among other things.
Based on the determined fault direction and estimated fault location, then, the relay 10 may decide that a detected fault is on the line 12 protected by such relay 10 or is otherwise within the xe2x80x98zone of protectionxe2x80x99 of such relay 10, and that an associated circuit breaker 18 should be tripped to isolate the faulted line 12. Accordingly, such relay 10 issues an appropriate command to the circuit breaker 10.
Typically, the command is issued over a xe2x80x98TRIPxe2x80x99 output of the relay 10 (xe2x80x98TRIP 1xe2x80x99 in FIG. 1A) and is appropriately received as an input to the circuit breaker 18. The relay 10 may then reset the circuit breaker 18 (by appropriate means) after the relay 10 senses that the fault has been cleared, or after otherwise deciding or being ordered to do so. Notably, the relay 10 may control several circuit breakers 18 (only one being shown in FIG. 1A), hence the xe2x80x98TRIP 2xe2x80x99 output (additional xe2x80x98TRIPxe2x80x99 outputs not being shown in FIG. 1A). Additionally, the circuit breakers 18 may be arranged to control one or more specific phases of the line 12, rather than all of the phases of the line 12. Owing to the relatively large distances over which a power system can extend, the distance between a relay 10 and one or more of its associated circuit breakers 18 can be substantial. As a result, the outputs from the relay 10 may be received by the circuit breaker(s) 18 by way of any reasonable transmission method, including hard wire line, radio transmission, optical link, satellite link, the transmission lines 12 themselves, and the like. Moreover, the commands may be transmitted and received as packetized network signals or the like on a data network rather than as signals on dedicated lines.
As seen in FIGS. 1 and 2, a transmission line, and in particular a relatively long transmission line, is oftentimes series-compensated by the installation of series capacitance 20 in the form of one or more capacitors or banks of capacitors (a representative series capacitor CAP is shown) at or adjacent one end of such line or perhaps even toward the middle of the line. Benefits obtained thereby include increased power transfer capability, improved system stability, reduced system losses, improved voltage regulation, and better power flow regulation. However, such installation of series capacitance 20 introduces challenges to protection systems for both the series-compensated line and lines adjacent thereto.
Typically, and as best seen in FIG. 2, installed series capacitance 20 includes a metal oxide varistor (MOV) or other non-linear protection device in parallel with the series capacitance (CAP), which limits the voltage across the capacitance in a pre-defined manner. Additionally, a bypass breaker or bypass switch (SW) is installed in parallel with the series capacitance, which closes following operation of a series capacitance overload protection system (not shown). The closing of such breaker introduces a transient in the system as the breaker arcs and the impedance seen by the protective relay is altered. In particular, the quick response of the MOV and overload protection (the spark gap (SG) installed in parallel with the series capacitance) removes or reduces the capacitance (CAP) and limits the impact of the transient.
Protection of a power distribution system with one or more series compensated lines is considered to be one of the most difficult tasks both for relay designers and utility engineers. A protective relay should be designed to have a high level of security and dependability. A utility engineer should be able to set the protection properly. However, protection settings depend on prevailing system conditions and system configuration, and both may change significantly if series capacitance is present in the system. In particular, such changes result from, among other things, the fact that series capacitance installed within a power system introduces an impedance that must be taken into account when measuring line impedance. More particular, and as is relevant in the present disclosure, the series capacitance and the fact that such series capacitance can vary must be taken into account during determination of fault direction.
In the present invention, accurate fault direction determination is accomplished by compensating fault direction-detecting elements for added series capacitance. In particular, in the present invention, an electrical power system includes a transmission line for transmitting electrical power, series capacitance compensation series-coupled to the transmission line adjacent one end thereof, where the series compensation includes a capacitance having a value (xe2x88x92j XCAP), and a protective relay at the one end of the transmission line for monitoring line voltages and line currents on the transmission line.
Upon sensing a fault, an impedance Z of the line is calculated based on the monitored line voltages and line currents. The calculated impedance Z is adjusted according to the value of the capacitance of the series compensation (xe2x88x92j XCAP) to result in a modified impedance ZMOD, and the phasor angle of ZMOD is examined to determine the direction of the sensed fault. The fault is in a first direction if the phasor angle is between X and X+180 degrees, and is in a second direction opposite the first direction if the phasor angle is between X+180 and X+360 degrees.